Mammalian protein kinases are involved in the regulation of important cellular functions. Due to the fact that dysfunctions in protein kinase activity have been associated with several diseases and disorders, protein kinases are targets for drug development.
Mixed lineage kinases (MLKs) are MAPK kinase kinases that target JNK and p38 MAPK for activation in response to diverse stimuli that stress cells. As a result, the MLKs regulate a broad range of cellular processes. MLK3 is the most widely expressed MLK family member and is present in neurons. It is activated by GTPases of the Ras superfamily, such as Cdc42 and Rac, which trigger protein dimerization via a leucine zipper interface, resulting in auto-phosphorylation at Thr277 and Ser281 within the protein activation loop and subsequent activation of the enzyme.
Preclinical studies of the mixed lineage kinase (MLK) inhibitor CEP1347 have shown that this agent can protect neurons against a considerable range of insults, including exposure to the Alzheimer's peptide, Aβ. Studies using the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model of Parkinsonism have demonstrated the efficacy of CEP1347 in treating motor deficits and neuronal degeneration, and CEP1347-mediated neuroprotection has also been observed in an in vitro model for Parkinson's Disease, using methamphetamine-exposed human mesencephalic-derived neurons. This finding suggests that CEP1347 might also be protective in the context of neurologic complications such as HIV-associated dementia (HAD). In fact, Bodner et al. have shown that CEP1347 can protect primary rat hippocampal neurons as well as dorsal root ganglion neurons from the otherwise lethal effects of exposure to HIV-1 gp120. It has been determined that CEP1347 mediates this effect by inhibiting the activity of the mixed lineage kinase (MLK) family.
Maggirwar et al. recently examined the effect of Tat and gp120 on MLK3. Tat and gp120 were shown to induce autophosphorylation of MLK3 in primary rat neurons and this was abolished by the addition of CEP1347. These studies suggest that the normal function of MLK3 is compromised by HIV-1 neurotoxins, resulting in the downstream signaling events that result in neuronal death and monocyte activation (with release of inflammatory cytokines). Most recently, Eggert et al. have demonstrated that CEP1347 is neuroprotective in an in vivo model of HIV-1 infection, reversing microglial activation and restoring normal synaptic architecture, as well as restoring macrophage secretory profiles to a trophic vs. toxic phenotype in response to HIV-1 infection. Eggert, D., Gorantla, S., Poluekova, L., Dou, H., Schifitto, G., Maggirwar, S. B., Dewhurst, S., Gelbard, H. A. and H. E. Gendelman: “Neuroprotective Activities of CEP-1347 in Models of HIV-1 Encephalitis,” J. Immunol. (in press).]
Recently, MLK3 has been shown to drive the production of the HIV virus. As a result, several lines of evidence now support that an inhibitor of MLK3 could serve as a treatment for numerous neurological conditions, including neuroAIDS. CEP1347 does not have ideal pharmacokinetic properties, which could potentially affect its ability to gain entry to the CNS. Other small molecule MLK3 inhibitors are needed that have improved pharmacokinetic and brain penetrating properties.
An inhibitor of MLK3 could also find use in the treatment of psychological disorders. Depression is a complex disease that has a multifactorial etiology. This may include genetic factors, changes in normal neuronal signaling, and reduced levels of certain neurotrophins (such as brain-derived neurotrophic factor, BDNF) within particular regions of the brain (Krishnan, V., and E. J. Nestler. 2008. Nature 455:894-902). Treatments for depression include drugs such as SSRIs, as well as cognitive and behavioral therapy (“talk therapy”) and other inventions such as exercise. Interestingly, SSRIs and exercise share the common property that they promote neurogenesis; this is thought to be related to their anti-depressive effects because of effects on neuronal plasticity and remodeling (Krishnan, supra).
Pharmacologic blockade of mixed lineage kinase 3 (MLK3) has been shown to result in activation of neurotrophin-mediated signaling pathways, and increased expression of neurotrophin receptors—resulting in enhanced responsiveness to endogenous neurotrophins, including BDNF (Wang, L. H., A. J. Paden, and E. M. Johnson, Jr. 2005. J Pharmacol Exp Ther 312:1007-19). MLK3 inhibitors have also been shown to increase production of BDNF itself (Conforti, P. et al. 2008. Mol Cell Neurosci 39:1-7).
Combined treatment with SSRIs and MLK3 inhibitors could result in the synergistic promotion of neurogenesis, due to the neurotrophin-sensitizing effects of MLK3 inhibitors and their ability to directly upregulate BDNF (Wang and Conforti, supra). Increase of the therapeutic effectiveness of SSRIs (and possibly talk therapy and exercise also) could also result if the compounds were coadministered.
Exposure to MLK3 inhibitors may also compensate for lowered BDNF levels in hippocampus of persons with depression, thereby alleviating depression (based on the “BDNF hypothesis”) (Krishnan, supra).